The monitoring of fluid flow is important in a wide variety of fields. An application of particular interest has been determining the volumetric throughput of paint in spray painting. A number of methods for such monitoring have been proposed in the past including devices mechanically moved by the streaming fluid such as paddle wheels and the measurement of the transit time of tracers such as heat pulses injected into streaming fluid. However, the precision and dependability of these methods has been found to be inadequate in certain applications. For instance, in spray painting with two component lacquer systems such as polyurethanes, it is important to control the ratio of the two components within narrow limits or a serious loss of quality may be experienced. To achieve this control, it is important to very precisely monitor the flow of one component and adjust the flow of the other component accordingly. Prior art monitoring processes have been found to be insufficiently accurate or reproducible particularly in the low output rates (50 to 200 cm.sup.3 /min) encountered in spray lacquering and coating.
It is an object of this invention to provide a method of monitoring the streaming velocity in fluid systems which is both accurate and reproducible. It is a further object to provide a method which can determine the streaming velocity very quickly allowing almost continuous monitoring.